Prop Entities, available only in the Source SDK, are (usually) inanimate objects that use a model for their appearance and function. They can be static, dynamic, or physics. Some more specific props can even have functionality, such as a prop_door_rotating. Examples of prop entities include prop_detail, prop_dynamic and prop_ragdoll.

NPC entities, or "Non-Player Characters" entities, are computer controlled characters and creatures that can interact with the player(s) using their AI coding. NPCs generally fall into two categories, Friendly, NPCs that help, assist, or do not affect the player, and Hostile, NPCs that attack or negatively affect the player.